The present invention relates to a computed tomography apparatus which takes an image of an object and can reduce the burden of an operator who makes an imaging plan.
An example of a computed tomography apparatus is an X-ray computed-tomography apparatus (hereafter called X-ray CT apparatus). A specific implementation of an X-ray CT apparatus, a multi-slice X-ray CT apparatus has been developed and has found widespread use in recent years. A multi-slice X-ray CT apparatus has a 2-dimensional detector including M channels of N segments detection elements where a plurality of channel detection elements are arranged along the segment direction perpendicular to the channel direction. A multi-slice X-ray CT apparatus collects an image which is characterized by having high resolution and wide range. Examples of multi-slice X-ray CT apparatus include 4 slice type, 8 slice type, and 16 slice type devices . . .
An example of a reconstructing method used with the multi-slice X-ray CT apparatus is a fan-beam reconstruction method which reconstructs the image on the assumption an X-ray beam is parallel to a direction perpendicular to a slice direction although the X-ray beam is, to be exact, a cone-like X-ray beam (cone-beam) which spreads in the slice direction. Another example of a reconstructing method is a cone-beam reconstruction method which reconstructs the image on basis of the angle of the cone-beam. The cone-beam reconstruction method is used when the number of slices to be simultaneously detected equal 8 and the fan-beam reconstruction method is used when the number of slices equal 4.
There are merits and demerits in these reconstruction methods, respectively. For example, the cone-beam reconstruction method makes excellent quality images but requires a longer reconstruction time as compared with the fan-beam reconstruction method because of the need to account for the cone angle. Thus, it is necessary for an operator to understand the special features of each of these reconstruction methods when choosing an appropriate reconstruction method for each patient and for specific images of patients. Setting up the equipment between shots with different methods is very difficult for an operator. Even if the operator is well-skilled in setting up the equipment, this set-up process takes much time and the patient processing efficiency (patient throughput) decreases. Although conventional imaging plan systems configured to assist the operator are known, they do not urge the operator to determine the reconstruction method according to scanning conditions.
In addition, there is another factor which causes a decrease in patient throughput. Another example of a multi-slice X-ray CT apparatus is a multi-slice (e.g., 4 slice or 8 slice) helical X-ray CT apparatus which performs a helical scan. With a multi-slice helical apparatus, the operator can choose the image slice width, thereby creating an imaging plan. The image slice width is defined as the number of imaging slices times the thickness of an imaging slice. The image slice width is also called a scan slice. The thickness of the imaging slice is defined by the thickness of the slice in a rotation center position and desired value is selected (e.g., from 0.5 mm, 1 mm, 2 mm, 3 mm and 4 mm). Thus, the number of imaging slices is the number of detection element segments corresponding to the thickness of the imaging slice (e.g., 0.5 mm).
There are a number of limitations associated with a conventional multi-slice helical X-ray CT apparatus. For example, after the operator sets that the number of imaging slices (e.g., =8) and a helical pitch (e.g., =7), before imaging, the operator may change the number of imaging slices (e.g., from 8 to 4) in response to various demands (quality of image, imaging speed, etc.) changes. It is possible in this case for an object to be imaged with the number of slices=4 and the helical pitch remaining equal to 7 if the operator forgets to change the helical pitch. As a result, an artifact will appear on the reconstructed image. (Note helical pitch is defined as the distance of the movement of the X-ray beam along the rotation axis when it makes a turn around the patient divided by the thickness of the imaging slice.) If the image many such artifacts, it is necessary to re-image the patient and patient throughput decreases.